In general, the purpose of a vaccine is to induce an immune response in the recipient, thus providing protection against subsequent challenge with a pathogen. This may be achieved by inoculation with a live attenuated strain of the pathogen, ie. a strain having reduced virulence such that it does not cause the disease caused by the virulent pathogen while still stimulating a broad immune response.
Enterotoxigenic E. coli (ETEC) strains are a major cause of travellers diarrhoea and of morbidity and death of children in endemic areas. Virulence is associated with expression of fimbrial colonisation factor antigens (CFAs) which mediate adhesion to the intestine and with secretion of toxins (heat stable toxin (ST), heat labile toxin (LT) and EAST toxin) which are responsible for the loss of fluid characteristic of the disease. Protection against ETEC disease is associated with antibody-mediated neutralisation of the toxins and with a humoral immune response against the CFAs.